Jet streams are fast-moving air currents high in the atmosphere that shape our weather patterns. They form due to temperature differences between the equator and poles, influenced by Earth's rotation. Understanding jet streams helps predict weather systems and their movements.

Two main jet streams exist in each hemisphere: the polar and subtropical. These powerful winds steer storms, influence temperatures, and can cause extreme weather events. Jet streams interact with Rossby waves, creating complex atmospheric patterns that affect global climate.

Jet Streams and Their Influence on Weather

Jet streams and their formation

• Narrow, fast-moving air currents in the upper troposphere and lower stratosphere
  • Occur at altitudes of 9-16 km (30,000-50,000 ft)
  • Wind speeds can exceed 400 km/h (250 mph) (hurricane-force winds)
• Formation driven by Coriolis force and temperature gradients
  • Coriolis force caused by Earth's rotation deflects moving objects to the right in N. Hemisphere and left in S. Hemisphere
  • Temperature gradients exist between equator (warm) and poles (cold)
  • Combination of Coriolis force and temperature gradients leads to geostrophic wind balance between pressure gradient force and Coriolis force
• Influenced by thermal wind perpendicular to temperature gradient with cold air to left and warm air to right in N. Hemisphere (opposite in S. Hemisphere)
  • Relationship causes jet streams to form at boundaries between cold and warm air masses (fronts)

Major jet stream locations

• Two main jet streams in each hemisphere: polar jet stream and subtropical jet stream
• Polar jet stream
  • Located between 50°-60° latitude in both hemispheres (over Canada and Russia in N. Hemisphere)
  • Separates cold polar air from warmer mid-latitude air
  • Stronger and more persistent in winter when temperature contrasts are greatest
• Subtropical jet stream
  • Located around 30° latitude in both hemispheres (over southern US and northern Africa in N. Hemisphere)
  • Associated with poleward edge of Hadley cell and descending branch of Ferrel cell
  • Strongest in winter hemisphere

Jet stream influence on weather

• Play crucial role in steering weather systems and influencing their development and intensity
• Mid-latitude cyclones (low-pressure systems) and anticyclones (high-pressure systems) tend to follow path of polar jet stream
  • Speed and direction of jet stream determine movement and intensity of these systems
  • Divergence in upper-level winds associated with jet streams can lead to surface cyclogenesis (formation of low-pressure systems)
• Jet streaks (localized areas of high wind speed within jet stream) can influence development and intensity of weather systems
  • Divergence in left exit and right entrance regions of jet streak can lead to rising motion, development of low-pressure systems, and precipitation (snowstorms)
  • Convergence in right exit and left entrance regions can lead to sinking motion, development of high-pressure systems, and clear skies
• Changes in position and strength of jet streams can lead to changes in temperature and precipitation patterns across large regions (droughts, heat waves)

Jet streams vs Rossby waves

• Rossby waves are large-scale meanders in the jet stream caused by Earth's rotation and variations in Coriolis force with latitude
• Formation of Rossby waves related to conservation of absolute vorticity (sum of Earth's vorticity due to rotation and relative vorticity of air)
  1. As air parcel moves poleward, Earth's vorticity increases, so air parcel must develop counterclockwise (cyclonic) relative vorticity to conserve absolute vorticity
  2. As air parcel moves equatorward, Earth's vorticity decreases, so air parcel must develop clockwise (anticyclonic) relative vorticity
• Process leads to formation of troughs (elongated areas of low pressure) and ridges (elongated areas of high pressure) in jet stream
• Wavelength and amplitude of Rossby waves determined by factors such as strength of jet stream, latitude, and variation in Coriolis force with latitude
  • Longer wavelengths and larger amplitudes associated with slower-moving, more meandering jet streams
  • Shorter wavelengths and smaller amplitudes associated with faster, more zonal (west-to-east) jet streams
• Rossby waves play key role in transfer of energy and momentum in atmosphere and can influence development and movement of weather systems on global scale (extreme weather events)